Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide a method and an apparatus including processing circuitry for video decoding. The processing circuitry is configured to decode coding information for a current block. The coding information can indicate that the current block is coded in a string copy mode. The processing circuitry is configured to determine a string vector (SV) and a string length of a current string in the current block based on the coding information. The string length can be N3 times L where N3 and L are positive integers, and L is larger than 1. The processing circuitry is configured to reconstruct the current string based on the SV and the string length of the current string. In an example, the current block is a luma block and L is 4.

INCORPORATION BY REFERENCE

This present disclosure claims the benefit of priority to U.S.Provisional Application No. 63/003,665, “HISTORY BASED VECTOR PREDICTIONFOR INTRA PICTURE BLOCK AND STRING COPYING” filed on Apr. 1, 2020, whichis incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has specific bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth and/or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless compression and lossy compression, as well as a combinationthereof can be employed. Lossless compression refers to techniques wherean exact copy of the original signal can be reconstructed from thecompressed original signal. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is smallenough to make the reconstructed signal useful for the intendedapplication. In the case of video, lossy compression is widely employed.The amount of distortion tolerated depends on the application; forexample, users of certain consumer streaming applications may toleratehigher distortion than users of television distribution applications.The compression ratio achievable can reflect that: higherallowable/tolerable distortion can yield higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is using reference data only from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y. or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 2 , a current block (201) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (202 through 206, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes processing circuitry. The processing circuitry can beconfigured to decode coding information for a current block of a currentpicture. The coding information can indicate a coding mode for thecurrent block being one of: an intra block copy (IBC) mode and a stringcopy mode. The processing circuitry can be configured to determinecurrent vector information for a current unit of samples in the currentblock based on the coding mode for the current block and a historybuffer. The history buffer can be configured to store vector informationof at least a block previously decoded in the IBC mode and a stringpreviously decoded in the string copy mode. The processing circuitry canbe configured to reconstruct the current unit of samples based on thecurrent vector information.

In an embodiment, the coding mode for the current block is the IBC mode.The current unit of samples is the current block. The processingcircuitry can be configured to determine a BV predictor candidate listfor the current block based at least on the vector information in thehistory buffer and determine, based on the BV predictor candidate list,a current BV in the current vector information.

In an embodiment, the coding mode for the current block is the stringcopy mode. The current unit of samples is a current string in thecurrent block. The processing circuitry can be configured to determine acurrent SV in the current vector information from the vector informationin the history buffer. The current SV is for the current string.

In an embodiment, the processing circuitry can be configured to storethe current vector information into the history buffer if the currentvector information is different from one or more of the vectorinformation in the history buffer. In an example, the processingcircuitry is configured to determine that the current vector informationis different from the one or more of the vector information in thehistory buffer when a difference between a current vector in the currentvector information and each previous vector of the one or more of thevector information in the history buffer is larger than a pre-determinedthreshold.

In an example, the processing circuitry can be configured to determinethat the current vector information is different from the one or more ofthe vector information in the history buffer based on a size differencebetween a current unit size of the current vector information and aprevious unit size of each of the one or more of vector informationbeing larger than a pre-determined size threshold. The current unit sizeof the current vector information can indicate a number of samples inthe current unit. The vector information in the history buffer can beused to decode previous units of samples that include the blockpreviously decoded in the IBC mode and the string previously decoded inthe string copy mode. The previous unit size can indicate a number ofsamples in the respective previous unit. In an example, based on adifference between the current vector and a previous vector of one ofthe one or more of the vector information not being larger than apre-determined threshold, the processing circuitry is configured toremove the one of the one or more of the vector information from thehistory buffer.

In an embodiment, the one or more of the vector information includes (i)a subset of the vector information in the history buffer or (ii) thevector information in the history buffer.

In an embodiment, one of the vector information in the history bufferincludes a string vector and one of (i) a string location and (ii) astring size of the string previously decoded in the string copy mode.The string location is a location of a pre-determined sample in thestring previously decoded in the string copy mode. The string size is anumber of samples in the string previously decoded in the string copymode.

In an embodiment, the vector information in the history buffer includesprevious vectors and previous unit sizes, and previous unit locations ofcorresponding previously decoded unit of samples that include the blockpreviously decoded in the IBC mode and the string previously decoded inthe string copy mode. The processing circuitry can be configured toclassify each of the vector information stored in the history bufferinto one of a plurality of categories based on at least one of: (i) theprevious unit size of the respective vector information, (ii) theprevious unit location of the respective vector information, or (iii) anumber of times that the respective vector information is used topredict one or more previously decoded units of samples. The historybuffer can be a class-based history buffer. In an example, the codinginformation for the current block further includes an index. Theprocessing circuitry can be configured to determine the current vectorinformation to be a first entry in one of the plurality of categoriesindicated by the index.

According to aspects of the disclosure, the processing circuitry can beconfigured to decode coding information for a current block. The codinginformation can indicate that the current block is coded in a stringcopy mode. The processing circuitry can be configured to determine astring vector (SV) and a string length of a current string in thecurrent block based on the coding information. The string length can beN3 times L where N3 and L are positive integers, and L is larger than 1.The processing circuitry can be configured to reconstruct the currentstring based on the SV and the string length of the current string.

In an example, the current block is a luma block and L is 4.

In an example, the current block is a chroma block. A chroma subsamplingformat is 4:2:0 indicating that the chroma block has half a height andhalf a width of a corresponding luma block. L is 2 based on the chromablock being coded jointly with the corresponding luma block, and L is 4based on the chroma block being coded separately from the correspondingluma block.

In an example, the current block includes one or more strings that hasthe current string. The current block further includes escape samplesoutside of the one or more strings. A number of the escape samples isone or a multiple of L. In an example, a number of escape samples in asame row of the current block is one or a multiple of L.

In an embodiment, the coding information further includes a syntaxelement indicating the string length. A coded value of the syntaxelement is the string length divided by L. The coded value of the syntaxelement is an integer in a range from 1 to (M1/L−1) where M1 is a numberof samples in the current block. In an example, the processing circuitryis configured to decode the syntax element and further determine thestring length to be the coded value of the syntax element multiplied byL.

In an embodiment, the processing circuitry is configured to decode asyntax element that indicates a resolution used for the SV. In anexample, the syntax element has 1 bit indicating that the resolution forthe SV is one of (i) 1-sample and (ii) 4-sample. Based on the resolutionfor the SV being 4-sample, the processing circuitry is configured todetermine an intermediate SV from the coding information and determinethe SV to be the intermediate SV multiplied by 4.

In an example, the current block includes a plurality of strings thathas the current string and a last string length of a last string that isto be coded in the plurality of strings is not signaled. In an example,the last string length is determined based on (i) a number of samples inthe current block and (ii) one or more string lengths of one or moreremaining strings in the plurality of strings. In an example, the codinginformation includes a flag indicating whether the current string is thelast string.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video encoding/decoding cause the computer to perform anyof the methods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes.

FIG. 1B is an illustration of exemplary intra prediction directions.

FIG. 2 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure.

FIG. 10 shows an example of intra block copy according to an embodimentof the disclosure.

FIG. 11 shows an example of intra block copy according to an embodimentof the disclosure.

FIGS. 12A-12D show examples of intra block copy according to anembodiment of the disclosure.

FIG. 13 shows an example of spatial classes for intra block copy blockvector prediction for a current block according to an embodiment of thedisclosure.

FIG. 14 shows an example of a string copy mode according to anembodiment of the disclosure.

FIG. 15 shows a flow chart outlining a process (1500) according to anembodiment of the disclosure.

FIG. 16 shows a flow chart outlining a process (1600) according to anembodiment of the disclosure.

FIG. 17 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (404) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412)(e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (501), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512)(e.g., a display screen) that is not an integral partof the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5 . The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler/inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640)(e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any color space (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630)(e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5 . Brieflyreferring also to FIG. 5 , however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously coded picture fromthe video sequence that were designated as “reference pictures.” In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6 ), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.11.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEVC example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in FIG. 7 .

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (722) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intramode, the general controller (721) controls the switch (726) to selectthe intra mode result for use by the residue calculator (723), andcontrols the entropy encoder (725) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(721) controls the switch (726) to select the inter prediction resultfor use by the residue calculator (723), and controls the entropyencoder (725) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (703) also includes a residuedecoder (728). The residue decoder (728) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (722) and theinter encoder (730). For example, the inter encoder (730) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (722) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (725) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8 .

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (872) or the inter decoder (880), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (880); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (872). The residual information can be subject to inversequantization and is provided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603),and (703), and the video decoders (410), (510), and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for history-based vectorprediction for an intra picture block, a string using a string copyingmode, or the like.

Block based compensation can be used for inter prediction and intraprediction. For the inter prediction, block based compensation from adifferent picture is known as motion compensation. Block basedcompensation can also be done from a previously reconstructed areawithin the same picture, such as in intra prediction. The block basedcompensation from reconstructed area within the same picture is referredto as intra picture block compensation, current picture referencing(CPR), or intra block copy (IBC). A displacement vector that indicatesan offset between a current block and a reference block (also referredto as a prediction block) in the same picture is referred to as a blockvector (BV) where the current block can be encoded/decoded based on thereference block. Different from a motion vector in motion compensation,which can be at any value (positive or negative, at either x or ydirection), a BV has a few constraints to ensure that the referenceblock is available and already reconstructed. Also, in some examples,for parallel processing consideration, some reference area that is tileboundary, slice boundary, or wavefront ladder shape boundary isexcluded.

The coding of a block vector could be either explicit or implicit. Inthe explicit mode, a BV difference between a block vector and itspredictor is signaled. In the implicit mode, the block vector isrecovered from a predictor (referred to as block vector predictor)without using the BV difference, in a similar way as a motion vector inthe merge mode. The explicit mode can be referred to as a non-merge BVprediction mode. The implicit mode can be referred to as a merge BVprediction mode.

The resolution of a block vector, in some implementations, is restrictedto integer positions. In other systems, the block vector is allowed topoint to fractional positions.

In some examples, the use of intra block copy at a block level can besignaled using a block level flag, such as an IBC flag. In anembodiment, the block level flag is signaled when the current block iscoded explicitly. In some examples, the use of intra block copy at ablock level can be signaled using a reference index approach. Thecurrent picture under decoding is then treated as a reference picture ora special reference picture. In an example, such a reference picture isput in the last position of a list of reference pictures. The specialreference picture is also managed together with other temporal referencepictures in a buffer, such as a decoded picture buffer (DPB).

There can be variations for the IBC mode. In an example, the IBC mode istreated as a third mode that is different from the intra prediction modeand the inter prediction mode. Accordingly, the BV prediction in theimplicit mode (or the merge mode) and the explicit mode are separatedfrom the regular inter mode. A separate merge candidate list can bedefined for the IBC mode where entries in the separate merge candidatelist are BVs. Similarly, in an example, a BV prediction candidate listin the IBC explicit mode only includes BVs. General rules applied to thetwo lists (i.e., the separate merge candidate list and the BV predictioncandidate list) are that the two lists may follow the same logic as amerge candidate list used in the regular merge mode or an AMVP predictorlist used in the regular AMVP mode in terms of the candidate derivationprocess. For example, the five spatial neighboring locations (e.g., A0,A1, and B0, B1, B2 in FIG. 2 ), for example, HEVC or VVC inter mergemode are accessed for the IBC mode to derive the separate mergecandidate list for the IBC mode.

As described above, a BV of a current block under reconstruction in apicture can have certain constraints, and thus, a reference block forthe current block is within a search range. The search range refers to apart of the picture from which the reference block can be selected. Forexample, the search range may be within certain portions of areconstructed area in the picture. A size, a position, a shape, and/orthe like of the search range can be constrained. Alternatively, the BVcan be constrained. In an example, the BV is a two-dimensional vectorincluding an x and a y component, and at least one of the x and ycomponents can be constrained. Constraints can be specified with respectto the BV, the search range, or a combination of the BV and the searchrange. In various examples, when certain constraints are specified withrespect to the BV, the search range is constrained accordingly.Similarly, when certain constraints are specified with respect to thesearch range, the BV is constrained accordingly.

FIG. 9 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (900) is to be reconstructed underdecoding. The current picture (900) includes a reconstructed area (910)(grey area) and a to-be-decoded area (920) (white area). A current block(930) is under reconstruction by a decoder. The current block (930) canbe reconstructed from a reference block (940) that is in thereconstructed area (910). A position offset between the reference block(940) and the current block (930) is referred to as a block vector (950)(or BV (950)). In the FIG. 9 example, a search range (960) is within thereconstructed area (910), the reference block (940) is within the searchrange (960), and the block vector (950) is constrained to point to thereference block (940) within the search range (960).

Various constraints can be applied to a BV and/or a search range. In anembodiment, a search range for a current block under reconstruction in acurrent CTB is constrained to be within the current CTB.

In an embodiment, an effective memory requirement to store referencesamples to be used in intra block copy is one CTB size. In an example,the CTB size is 128×128 samples. A current CTB includes a current regionunder reconstruction. The current region has a size of 64×64 samples.Since a reference memory can also store reconstructed samples in thecurrent region, the reference memory can store 3 more regions of 64×64samples when a reference memory size is equal to the CTB size of 128×128samples. Accordingly, a search range can include certain parts of apreviously reconstructed CTB while a total memory requirement forstoring reference samples is unchanged (such as 1 CTB size of 128×128samples or 4 64×64 reference samples in total). In an example, thepreviously reconstructed CTB is a left neighbor of the current CTB, suchas shown in FIG. 10 .

FIG. 10 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (1001) includes a current CTB(1015) under reconstruction and a previously reconstructed CTB (1010)that is a left neighbor of the current CTB (1015). CTBs in the currentpicture (1001) have a CTB size, such as 128×128 samples, and a CTBwidth, such as 128 samples. The current CTB (1015) includes 4 regions(1016)-(1019), where the current region (1016) is under reconstruction.The current region (1016) includes a plurality of coding blocks(1021)-(1029). Similarly, the previously reconstructed CTB (1010)includes 4 regions (1011)-(1014). The coding blocks (1021)-(1025) arereconstructed, the current block (1026) is under reconstruction, and thecoding blocks (1026)-(1027) and the regions (1017)-(1019) are to bereconstructed.

The current region (1016) has a collocated region (i.e., the region(1011), in the previously reconstructed CTB (1010)). A relative positionof the collocated region (1011) with respect to the previouslyreconstructed CTB (1010) can be identical to a relative position of thecurrent region (1016) with respect to the current CTB (1015). In theexample illustrated in FIG. 10 , the current region (1016) is atop leftregion in the current CTB (1015), and thus, the collocated region (1011)is also a top left region in the previously reconstructed CTB (1010).Since a position of the previously reconstructed CTB (1010) is offsetfrom a position of the current CTB (1015) by the CTB width, a positionof the collocated region (1011) is offset from a position of the currentregion (1016) by the CTB width.

In an embodiment, a collocated region of the current region (1016) is ina previously reconstructed CTB where a position of the previouslyreconstructed CTB is offset by one or multiples of the CTB width fromthe positon of the current CTB (1015), and thus, a position of thecollocated region is also offset by a corresponding one or multiples ofthe CTB width from the position of the current region (1016). Theposition of the collocated region can be left shifted, up shifted, orthe like from the current region (1016).

As described above, a size of a search range for the current block(1026) is constrained by the CTB size. In the FIG. 10 example, thesearch range can include the regions (1012)-(1014) in the previouslyreconstructed CTB (1010) and a portion of the current region (1016) thatis already reconstructed, such as the coding blocks (1021)-(1025). Thesearch range further excludes the collocated region (1011) so that thesize of the search range is within the CTB size. Referring to FIG. 10 ,a reference block (1091) is located in the region (1014) of thepreviously reconstructed CTB (1010). A block vector (1020) indicates anoffset between the current block (1026) and the respective referenceblock (1091). The reference block (1091) is in the search range.

The example illustrated in FIG. 10 can be suitably adapted to otherscenarios where a current region is located at another location in thecurrent CTB (1015). In an example, when a current block is in the region(1017), a collocated region for the current block is the region (1012).Therefore, a search range can include the regions (1013)-(1014), theregion (1016), and a portion of the region (1017) that is alreadyreconstructed. The search range further excludes the region (1011) andthe collocated region (1012) so that the size of the search range iswithin the CTB size. In an example, when a current block is in theregion (1018), a collocated region for the current block is the region(1013). Therefore, a search range can include the region (1014), theregions (1016)-(1017), and a portion of the region (1018) that isalready reconstructed. The search range further excludes the regions(1011)-(1012) and the collocated region (1013) so that the size of thesearch range is within the CTB size. In an example, when a current blockis in the region (1019), a collocated region for the current block isthe region (1014). Therefore, a search range can include the regions(1016-(1018), and a portion of the region (1019) that is alreadyreconstructed. The search range further excludes the previouslyreconstructed CTB (1010) so that the size of the search range is withinthe CTB size.

In the above description, a reference block can be in the previouslyreconstructed CTB (1010) or the current CTB (1015).

In an embodiment, a search range can be specified as below. In anexample, a current picture is a luma picture and a current CTB is a lumaCTB including a plurality of luma samples and a BV (mvL) satisfies thefollowing constraints for bitstream conformance. In an example, the BV(mvL) has a fractional resolution (e.g., 1/16-pel resolution).

The constraints include first conditions that a reference block for thecurrent block is already reconstructed. When the reference block has arectangular shape, a neighboring block availability checking process (ora reference block availability checking process) can be implemented tocheck whether a top left sample and a bottom right sample of thereference block are reconstructed. When both the top left sample and thebottom right sample of the reference block are reconstructed, thereference block is determined to be reconstructed.

For example, when a derivation process for reference block availabilityis invoked with a position (xCurr, yCurr) of a top left sample of thecurrent block set to be (xCb, yCb) and a position (xCb+(mvL[0]>>4),yCb+(mvL[1]>>4)) of the top left sample of the reference block asinputs, an output is equal to TRUE when the top left sample of thereference block is reconstructed where the block vector mvL is atwo-dimensional vector having a x component mvL[0] and a y componentmvL[1]. When the BV (mvL) has a fractional resolution, such as 1/16-pelresolution, the x component mvL[0] and the y component mvL[1] areshifted to have an integer resolution, as indicated by mvL[0]>>4 andmvL[1]>>4, respectively.

Similarly, when a derivation process for block availability is invokedwith the position (xCur, yCurr) of the top left sample of the currentblock set to be (xCb, yCb) and a position (xCb+(mvL[0]>>4)+cbWidth−1,yCb+(mvL[1]>>4)+cbHeight−1) of the bottom right sample of the referenceblock as inputs, an output is equal to TRUE when the bottom right sampleof the reference block is reconstructed. The parameters cbWidth andcbHeight represent a width and a height of the reference block.

The constraints can also include at least one of the following secondconditions: 1) a value of (mvL[0]>>4)+cbWidth is less than or equal to0, which indicates that the reference block is to the left of thecurrent block and does not overlap with the current block; 2) a value of(mvL[1]>>4)+cbHeight is less than or equal to 0, which indicates thatthe reference block is above the current block and does not overlap withthe current block.

The constraints can also include that the following third conditions aresatisfied by the block vector mvL:(yCb+(mvL[1]>>4))>>Ctb Log 2SizeY=yCb>>Ctb Log 2SizeY  (1)(yCb+(mvL[1]>>4+cbHeight−1)>>Ctb Log 2SizeY=yCb>>Ctb Log 2Size  (2)(xCb+(mvL[0]>>4))>>Ctb Log 2SizeY>=(xCb>>Ctb Log 2SizeY)−1  (3)(xCb+(mvL[0]>>4)+cbWidth−1)>>Ctb Log 2SizeY<=(xCb>>Ctb Log 2SizeY)  (4)where the parameters CtbLog2SizeY represents the CTB width in log 2form. For example, when the CTB width is 128 samples, CtbLog2SizeY is 7.Eqs. (1)-(2) specify that a CTB including the reference block is in asame CTB row as the current CTB (e.g., the previously reconstructed CTB(1010) is in a same row as the current CTB (1015) when the referenceblock is in the previously reconstructed CTB (1010)). Eqs. (3)-(4)specify that the CTB including the reference block is either in a leftCTB column of the current CTB or a same CTB column as the current CTB.The third conditions as described by Eqs. (1)-(4) specify that the CTBincluding the reference block is either the current CTB, such as thecurrent CTB (1015), or a left neighbor, such as the previouslyreconstructed CTB (1010), of the current CTB, similarly to thedescription with reference to FIG. 10 .

The constraints can further include fourth conditions: when thereference block is in the left neighbor of the current CTB, a collocatedregion for the reference block is not reconstructed (i.e., no samples inthe collocated region have been reconstructed). Further, the collocatedregion for the reference block is in the current CTB. In the FIG. 10example, a collocated region for the reference block (1091) is theregion (1019) that is offset by the CTB width from the region (1014)where the reference block (1091) is located and the region (1019) hasnot been reconstructed. Therefore, the block vector (1020) and thereference block (1091) satisfy the fourth conditions described above.

In an example, the fourth conditions can be specified as below: when(xCb+(mvL[0]>>4))>>CtbLog2SizeY is equal to (xCb>>CtbLog2SizeY)−1, thederivation process for reference block availability is invoked with theposition of the current block (xCurr, yCurr) set to be (xCb, yCb) and aposition(((xCb+(mvL[0]>>4)+CtbSizeY)>>(CtbLog2SizeY−1))>>(CtbLog2SizeY−1),((yCb+(mvL[1]>>4))<<(CtbLog2SizeY−1))<<(CtbLog2SizeY−1)) as inputs, anoutput is equal to FALSE indicating that the collocated region is notreconstructed, such as shown in FIG. 10 .

The constraints for the search range and/or the block vector can includea suitable combination of the first, second, third, and fourthconditions described above. In an example, the constraints include thefirst, second, third, and fourth conditions, such as shown in FIG. 10 .In an example, the first, second, third, and/or fourth conditions can bemodified and the constraints include the modified first, second, third,and/or fourth conditions.

According to the fourth conditions, when one of the coding blocks(1022)-(1029) is a current block, a reference block cannot be in theregion (1011), and thus, a search range for the one of the coding blocks(1022)-(1029) excludes the region (1011). The reasons why the region(1011) is excluded are specified below: if the reference block is in theregion (1011), then a collocated region for the reference block is theregion (1016), however, at least samples in the coding block (1021) havebeen reconstructed, and thus, the fourth conditions are violated. On theother hand, for a coding block to be reconstructed first in a currentregion, such as a coding block (1121) in a region (1116) in FIG. 11 ,the fourth conditions does not prevent a reference block to be in theregion (1111) because a collocated region (1116) for the reference blockhas not been reconstructed yet.

FIG. 11 shows an example of intra block copy according to an embodimentof the disclosure. A current picture (1101) includes a current CTB(1115) under reconstruction and a previously reconstructed CTB (1110)that is a left neighbor of the current CTB (1115). CTBs in the currentpicture (1101) have a CTB size and a CTB width. The current CTB (1115)includes 4 regions (1116)-(1119) where the current region (1116) isunder reconstruction. The current region (1116) includes a plurality ofcoding blocks (1121)-(1129). Similarly, the previously reconstructed CTB(1110) includes 4 regions (1111)-(1114). The current block (1121) underreconstruction is to be reconstructed first in the current region (1116)and the coding blocks (1122)-(1129) are to be reconstructed. In anexample, the CTB size is 128×128 samples, each of the regions(1111)(1114) and (1116)-(1119) is 64×64 samples. A reference memory sizeis equal to the CTB size and is 128×128 samples, and thus, the searchrange, when bounded by the reference memory size, includes 3 regions anda portion of an additional region.

Similarly as described with reference to FIG. 10 , the current region(1116) has a collocated region (i.e., the region (1111) in thepreviously reconstructed CTB (1110)). According to the fourth conditionsdescribed above, a reference block for the current block (1121) can bein the region (I 111), and thus, a search range can include the regions(1111)-(1114). For example, when the reference block is in the region(111), a collocated region of the reference block is the region (1116),where no samples in the region (1116) have been reconstructed prior tothe reconstruction of the current block (1121). However, as describedwith reference to FIG. 10 and the fourth conditions, for example, afterthe reconstruction of the coding block (1121), the region (1111) is nolonger available to be included in a search range for reconstructing thecoding block (1122). Therefore, a tight synchronization and timingcontrol of the reference memory buffer is to be used and can bechallenging.

According to some embodiments, when a current block is to bereconstructed first in a current region of a current CTB, a search rangecan exclude a collocated region of the current region that is in apreviously reconstructed CTB where the current CTB and the previouslyreconstructed CTB are in a same current picture. A block vector can bedetermined such that a reference block is in the search range thatexcludes the collocated region in the previously reconstructed CTB. Inan embodiment, the search range includes coding blocks that arereconstructed after the collocated region and before the current blockin a decoding order.

In the descriptions below, a CTB size can vary and a maximum CTB size isset to be identical to a reference memory size. In an example, thereference memory size or the maximum CTB size is 128×128 samples. Thedescriptions can be suitably adapted to other reference memory sizes ormaximum CTB sizes.

In an embodiment, the CTB size is equal to the reference memory size.The previously reconstructed CTB is a left neighbor of the current CTB,a position of the collocated region is offset by a CTB width from aposition of the current region, and the coding blocks in the searchrange are in at least one of: the current CTB and the previouslyreconstructed CTB.

FIGS. 12A-12D show examples of intra block copy according to anembodiment of the disclosure. Referring to FIGS. 12A-D, a currentpicture (1201) includes a current CTB (1215) under reconstruction and apreviously reconstructed CTB (1210) that is a left neighbor of thecurrent CTB (1215), CTBs in the current picture (1201) have a CTB sizeand a CTB width. The current CTB (1215) includes 4 regions(1216)-(1219). Similarly, the previously reconstructed CTB (1210)includes 4 regions (1211)-(1214). In an embodiment, the CTB size is amaximum CTB size and is equal to a reference memory size. In an example,the CTB size and the reference memory size are 128 by 128 samples, andthus, each of the regions (1211)-(1214) and (1216)(1219) has a size of64 by 64 samples.

In the examples illustrated in FIGS. 12A-D, the current CTB (1215)includes a top left region, a top right region, a bottom left region,and a bottom right region that correspond to the regions (1216)-(1219),respectively. The previously reconstructed CTB (1210) includes a topleft region, a top right region, a bottom left region, and a bottomright region that correspond to the regions (1211)-(1214), respectively.

Referring to FIG. 12A, the current region (1216) is underreconstruction. The current region (1216) can include a plurality ofcoding blocks (1221)-(1229). The current region (1216) has a collocatedregion, i.e., the region (1211), in the previously reconstructed CTB(1210). A search range for one of the coding blocks (1221)-(1229) to bereconstructed can exclude the collocated region (1211). The search rangecan include the regions (1212)-(1214) of the previously reconstructedCTB (1210) that are reconstructed after the collocated region (1211) andbefore the current region (1216) in a decoding order.

Referring to FIG. 12A, a position of the collocated region (1211) isoffset by the CTB width, such as 128 samples, from a position of thecurrent region (1216). For example, the position of the collocatedregion (1211) is left shifted by 128 samples from the position of thecurrent region (1216).

Referring again to FIG. 12A, when the current region (1216) is the topleft region of the current CTB (1215), the collocated region (1211) isthe top left region of the previously reconstructed CTB (1210), and thesearch region excludes the top left region of the previouslyreconstructed CTB.

Referring to FIG. 12B, the current region (1217) is underreconstruction. The current region (1217) can include a plurality ofcoding blocks (1241)-(1249). The current region (1217) has a collocatedregion (i.e., the region (1212), in the previously reconstructed CTB(1210)). A search range for one of the plurality of coding blocks(1241)-(1249) can exclude the collocated region (1212). The search rangeincludes the regions (1213)-(1214) of the previously reconstructed CTB(1210) and the region (1216) in the current CTB (1215) that arereconstructed after the collocated region (1212) and before the currentregion (1217). The search range further excludes the region (1211) dueto constraint of the reference memory size (i.e., one CTB size).Similarly, a position of the collocated region (1212) is offset by theCTB width, such as 128 samples, from a position of the current region(1217).

In the FIG. 12B example, the current region (1217) is the top rightregion of the current CTB (1215), the collocated region (1212) is alsothe top right region of the previously reconstructed CTB (1210), and thesearch region excludes the top right region of the previouslyreconstructed CTB (1210).

Referring to FIG. 12C, the current region (1218) is underreconstruction. The current region (1218) can include a plurality ofcoding blocks (1261)-(1269). The current region (1218) has a collocatedregion (i.e., the region (1213)), in the previously reconstructed CTB(1210). A search range for one of the plurality of coding blocks(1261)-(1269) can exclude the collocated region (1213). The search rangeincludes the region (1214) of the previously reconstructed CTB (1210)and the regions (1216)-(1217) in the current CTB (1215) that arereconstructed after the collocated region (1213) and before the currentregion (1218). Similarly, the search range further excludes the regions(1211)-(1212) due to constraint of the reference memory size. A positionof the collocated region (1213) is offset by the CTB width, such as 128samples, from a position of the current region (1218). In the FIG. 12Cexample, when the current region (1218) is the bottom left region of thecurrent CTB (1215), the collocated region (1213) is also the bottom leftregion of the previously reconstructed CTB (1210), and the search regionexcludes the bottom left region of the previously reconstructed CTB(1210).

Referring to FIG. 12D, the current region (1219) is underreconstruction. The current region (1219) can include a plurality ofcoding blocks (1281)-(1289). The current region (1219) has a collocatedregion (i.e., the region (1214)), in the previously reconstructed CTB(1210). A search range for one of the plurality of coding blocks(1281)-(1289) can exclude the collocated region (1214). The search rangeincludes the regions (1216)-(1218) in the current CTB (1215) that arereconstructed after the collocated region (1214) and before the currentregion (1219) in a decoding order. The search range excludes the regions(1211)-(1213) due to constraint of the reference memory size, and thus,the search range excludes the previously reconstructed CTB (1210).Similarly, a position of the collocated region (1214) is offset by theCTB width, such as 128 samples, from a position of the current region(1219). In the FIG. 12D example, when the current region (1219) is thebottom right region of the current CTB (1215), the collocated region(1214) is also the bottom right region of the previously reconstructedCTB (1210) and the search region excludes the bottom right region of thepreviously reconstructed CTB (1210).

Referring back to FIG. 2 , the MVs associated with the five surroundingsamples (or positions), denoted A0, A1, and B0, B1, B2 (202 through 206,respectively) can be referred to as spatial merge candidates. Acandidate list (e.g., a merge candidate list) can be formed based on thespatial merge candidates. Any suitable order can be used to form thecandidate list from the positions. In an example, the order can be A0,B0, B1, A1, and B2 where A0 is the first and B2 is the last. In anexample, the order can be A1, B1, B0, A0, and B2 where A1 is the firstand B2 is the last.

According to some embodiments, motion information of previously codedblocks for a current block (e.g., a coding block (CB) or a current CU)can be stored in a history-based motion vector prediction (H MVP) buffer(e.g., a table) to provide motion vector prediction (MVP) candidates(also referred to as HMVP candidates) for the current block. The HMVPbuffer may include one or more HMVP candidates, and can be maintainedduring an encoding/a decoding process. In an example, a HMVP candidatein the HMVP buffer corresponds to the motion information of a previouslycoded block. The HMVP buffer can be used in any suitable encoder and/ordecoder. HMVP candidate(s) can be added to a merge candidate list afterspatial MVP(s) and TMVP(s).

The HMVP buffer can be reset (e.g., emptied) when a new CTU (or a newCTB) row is encountered. When there is a non-subblock inter-coded block,the associated motion information can be added to a last entry of theHMVP buffer as a new HMVP candidate.

In an example, such as in VTM3, a buffer size (denoted by S) of the HMVPbuffer is set to be 6, indicating that up to 6 HMVP candidates may beadded to the HMVP buffer. In some embodiments, the HMVP buffer mayoperate in a first-in-first-out (FIFO) rule, and thus, a piece of motioninformation (or a HMVP candidate) that is stored first in the HMVPbuffer is the first to be removed from the HMVP buffer, for example,when the HMVP buffer is full. When inserting a new HMVP candidate to theHMVP buffer, a constrained FIFO rule can be utilized where a redundancycheck is first applied to determine whether an identical or similar HMVPcandidate is in the HMVP buffer. If an identical or similar HMVPcandidate is determined to be in the HMVP buffer, the identical orsimilar HMVP candidate can be removed from the HMVP buffer and remainingHMVP candidates can be moved forward in the HMVP buffer.

The HMVP candidates can be used in a merge candidate list constructionprocess, for example, in a merge mode. The most recent stored HMVPcandidate(s) in the HMVP buffer can be checked in an order and insertedto the merge candidate list after the TMVP candidate(s). A redundancycheck can be applied to the HMVP candidates with respect to the spatialor temporal merge candidates that are in the merge candidate list. Thedescriptions can be suitably adapted to an AMVP mode to construct anAMVP candidate list.

To reduce a number of redundancy check operations, the followingsimplifications can be used.

(i) A number of HMVP candidates used for generating the merge candidatelist can be set as (N<=4)?M:(8−N). N indicates a number of existingcandidates in the merge candidate list and M indicates a number ofavailable HMVP candidate(s) in the HMVP buffer. When the number ofexisting candidates (N) in the merge candidate list is less than orequal to 4, the number of HMVP candidates used for generating the mergecandidate list equals to M. Otherwise, the number of HMVP candidatesused for generating the merge candidate list equals to (8−N).

(ii) When the total number of available merge candidates reaches themaximum allowed merge candidates minus 1, the merge candidate listconstruction process from the HMVP buffer is terminated.

When the IBC mode operates as a separate mode from the inter predictionmode, a simplified BV derivation process for the IBC mode can be used. Ahistory-based block vector prediction buffer (referred as a HBVP buffer)can be used to perform BV prediction. The HBVP buffer can be used forstoring BV information (e.g., BVs) of previously coded blocks of acurrent block (e.g., a CB or a CU) in a current picture. In an example,the HBVP buffer is a separate history buffer from other buffer(s), suchas a HMVP buffer. The HBVP buffer can be a table.

The HBVP buffer can provide BV predictor (BVP) candidates (also referredto as HBVP candidates) for the current block. The HBVP buffer (e.g., thetable) may include one or more HBVP candidates, and can be maintainedduring an encoding/a decoding process. In an example, a HBVP candidatein the HBVP buffer corresponds to the BV information of a previouslycoded block in the current picture. The HBVP buffer can be used in anysuitable encoder and/or decoder. HBVP candidate(s) can be added to amerge candidate list configured for BV prediction after BV(s) of spatialneighboring block(s) of the current block. The merge candidate listconfigured for BV prediction can be used for the merge BV predictionmode and/or the non-merge BV prediction mode.

The HBVP buffer can be reset (e.g., emptied) when a new CTU (or a newCTB) row is encountered.

In an example, such as in VVC, a buffer size of the HBVP buffer is setto be 6, indicating that up to 6 HBVP candidates may be added to theHBVP buffer. In some embodiments, the HBVP buffer may operate in theFIFO rule, and thus, a piece of BV information (or a HBVP candidate)that is stored first in the HBVP buffer is the first to be removed fromthe HBVP buffer, for example, when the HBVP buffer is full. Wheninserting a new HBVP candidate into the HBVP buffer, a constrained FIFOrule can be utilized where a redundancy check is first applied todetermine whether an identical or similar HBVP candidate is in the HBVPbuffer. If an identical or similar HBVP candidate is determined to be inthe HBVP buffer, the identical or similar HBVP candidate can be removedfrom the HBVP buffer and remaining HBVP candidates can be moved forwardin the HBVP buffer.

The HBVP candidates can be used in a merge candidate list constructionprocess, for example, in the merge BV prediction mode. The most recentstored HBVP candidate(s) in the HBVP buffer can be checked in an orderand inserted into the merge candidate list after the spatialcandidate(s). A redundancy check can be applied to the HBVP candidateswith respect to the spatial merge candidates that are in the mergecandidate list.

In an embodiment, a HBVP buffer is established to store one or morepieces of BV information of one or more previously coded blocks coded inthe IBC mode. The one or more pieces of BV information can include oneor more BVs of the one or more previously coded blocks coded in the IBCmode. Further, each of the one or more pieces of BV information caninclude side information (or additional information) such as a blocksize, a block location, and/or the like of the respective previouslycoded block coded in the IBC mode.

In class-based history-based block vector prediction (also referred toas CBVP), for the current block, one or more pieces of BV information inthe HBVP buffer that meet certain conditions can be classified intocorresponding categories (also referred to as classes), and thus forminga CBVP buffer. In an example, each piece of BV information in the HBVPbuffer is for a respective previously coded block, for example, codedwith the IBC mode. The piece of BV information for the previously codedblock can include a BV, a block size, a block position, and/or the like.The previously coded block has a block width, a block height, and ablock area. The block area can be a multiplication of the block widthand the block height. In an example, the block size is represented bythe block area. The block position of the previously coded block can berepresented by an upper left corner (e.g., an upper left corner of 4×4area) or an upper left sample of the previously coded block.

FIG. 13 shows an example of spatial classes for IBC BV prediction for acurrent block (e.g., a CB, a CU) (1310) according to an embodiment ofthe disclosure. A left region (1302) can be to the left of the currentblock (1310). BV information for previously coded block(s) havingrespective block position(s) in the left region (1302) can be referredto as left candidates or left BV candidates. A top region (1303) can beabove the current block (1310). BV information for previously codedblock(s) having respective block position(s) in the top region (1303)can be referred to as top candidates or top BV candidates. A top-leftregion (1304) can be to a top-left of the current block (1310). BVinformation for previously coded block(s) having respective blockposition(s) in the top-left region (1304) can be referred to as top-leftcandidates or top-left BV candidates. A top-right region (1305) can beto a top-right of the current block (1310). BV information forpreviously coded block(s) having respective block position(s) in thetop-right region (1305) can be referred to as top-right candidates ortop-right BV candidates. A bottom-left region (1306) can be to abottom-left of the current block (1310). BV information for previouslycoded block(s) having respective block position(s) in the bottom-leftregion (1306) can be referred to as bottom-left candidates orbottom-left BV candidates. Other kinds of spatial classes can also bedefined and used in the CBVP buffer.

If the BV information for the previously coded block meets the followingconditions, the BV information can be classified into the correspondingcategories (or classes).

-   -   (i) Class 0: the block size (e.g., the block area) is greater        than or equal to a threshold (e.g., 64 pixels).    -   (ii) Class 1: an occurrence (or a frequency) of the BV is        greater than or equal to 2. The occurrence of the BV can refer        to a number of times the BV is used to predict previously coded        block(s). When a pruning process is used to form the CBVP        buffer, the BV can be stored in one entry (instead of in        multiples entries having the same BV) when the BV is used        multiple times in predicting previously coded blocks. The        occurrence of the BV can be recorded.    -   (iii) Class 2: the block position is in the left region (1302)        where a portion (e.g., an upper left corner of 4×4 area) of the        previously coded block is to the left of the current block        (1310). The previously coded block can be within the left region        (1302). Alternatively, the previously coded block can span        across multiple regions including the left region (1302) where        the block position is in the left region (1302).    -   (iv) Class 3: the block position is in the top region (1303)        where a portion (e.g., the upper left corner of 4×4 area) of the        previously coded block is above the current block (1310). The        previously coded block can be within the top region (1303).        Alternatively, the previously coded block can span across        multiple regions including the top region (1303) where the block        position is in the top region (1303).    -   (v) Class 4: the block position is in the top-left region (1304)        where a portion (e.g., the upper left corner of 4×4 area) of the        previously coded block is at the top-left side of the current        block (1310). The previously coded block can be within the        top-left region (1304). Alternatively, the previously coded        block can span across multiple regions including the top-left        region (1304) where the block position is in the top-left region        (1304).    -   (vi) Class 5: the block position is in the top-right region        (1305) where a portion (e.g., the upper left corner of 4×4 area)        of the previously coded block is at the top-right side of the        current block (1310). The previously coded block can be within        the top-right region (1305). Alternatively, the previously coded        block can span across multiple regions including the top-right        region (1305) where the block position is in the top-right        region (1305).    -   (vii) Class 6: the block position is in the bottom-left region        (1306) where a portion (e.g., the upper left corner of 4×4 area)        of the coded block is at the bottom-left side of the current        block (1310). The previously coded block can be within the        bottom-left region (1306). Alternatively, the previously coded        block can span across multiple regions including the bottom-left        region (1306) where the block position is in the bottom-left        region (1306).

For each category (or class), the BV of the most recently coded blockcan be derived as the BVP candidate. The CBVP buffer can be constructedby appending the BV predictor(s) of each category in an order from Class0 to Class 6. The above description for the CBVP can be suitably adaptedto include less classes or additional classes not described above. Oneor more of the Classes 0-6 can be modified. In an example, each entry inthe HBVP buffer is classified into one of the seven Classes 0-6. Anindex can be signaled to indicate which of the Classes 0-6 is chosen. Ata decoder side, the first entry in the chosen class can be used topredict a BV for the current block.

FIG. 14 shows an example of a string copy mode according to anembodiment of the disclosure. The string copy mode can also be referredto as a string matching mode or an intra string copy mode. A currentpicture (1410) includes a reconstructed region (a grey area) (1420) anda region (1421) that is under reconstruction. A current block (1435) inthe region (1421) is under reconstruction. The current block (1435) canbe a CB, a CU, or the like. The current block (1435) can include aplurality of strings (e.g., strings (1430) and (1431)). In an example,the current block (1435) is divided into a plurality of continuousstrings where one string is followed by a next string along a scanorder. The scan order can be any suitable scan order, such as a rasterscan order, a traverse scan order, or the like.

The reconstructed region (1420) can be used as a reference area toreconstruct the strings (1430) and (1431).

For each of the plurality of strings, a string offset vector (referredto as an SV) and a length of the string (referred to as a string length)can be signaled. The SV (e.g., a SV0) can be a displacement vector thatindicates a displacement between the string (e.g., the string (1430)) tobe reconstructed and a respective reference string (e.g., a referencestring (1400)) that is located in the reference area (1420) alreadyreconstructed. The reference string can be used to reconstruct thestring to be reconstructed. Thus, the SV can indicate where thecorresponding reference string is located in the reference area (1420).The string length can also indicate a length of the reference string.Referring to FIG. 14 , the current block (1435) is an 8×8 CB including64 samples and is divided into two strings (e.g., the strings (1430) and(1431)) using the raster scan order. The string (1430) includes first 29samples of the current block (1435), and the string (1431) includesremaining 35 samples of the current block (1435). The reference string(1400) used to reconstruct the string (1430) can be indicated by acorresponding string vector SV0, and a reference string (1401) used toreconstruct the string (1431) can be indicated by a corresponding stringvector SV1.

In general, a string size can refer to a length of a string or a numberof samples in a string. Referring to FIG. 14 , the string (1430)includes 29 samples, and thus a string size of the string (1430) is 29.The string (1431) includes 35 samples, and thus a string size of thestring (1431) is 35. A string location (or a string position) can berepresented by a sample position of a sample (e.g., a first sample in adecoding order) in the string.

The above descriptions can be suitably adapted to reconstruct a currentblock that includes any suitable number of strings. Alternatively, in anexample, when a sample in a current block does not have a matchingsample in a reference area, an escape sample is signaled, and a value ofthe escape sample can be coded directly without referring to areconstructed sample in the reference area.

In the disclosure, vector prediction can include BV prediction in theIBC mode and SV prediction in the string copy mode. The vectorprediction can include a skip mode vector prediction, a merge mode (or adirect mode) vector prediction, or the vector prediction with adifference coding. In the skip mode vector prediction and the merge modevector prediction, a vector (e.g., a BV, a SV) can be recovered from apredictor (or a vector predictor, such as a BV predictor, an SVpredictor) directly without using a difference coding. For example, themerge mode vector prediction for the BV prediction is the merge BVprediction mode, and the BV can be equal to the BV predictor. Similarly,the merge mode vector prediction for the SV prediction is the merge SVprediction mode, and the SV can be equal to the SV predictor. The vectorprediction with a difference coding for the BV prediction is theexplicit mode or the non-merge BV prediction mode.

According to aspects of the disclosure, a history-based SV predictionbuffer (referred as a HSVP buffer) can be used to perform SV prediction.The HSVP buffer can be configured to store one or more pieces ofprevious SV information (e.g., SVs) of one or more previously codedstrings coded in the string copy mode of a current picture. The HSVPbuffer can be used to predict a current string in a current block of thecurrent picture. In an example, the one or more previously coded stringscoded in the string copy mode are decoded prior to the current block.The one or more pieces of previous SV information can include one ormore previous SVs of the one or more previously coded strings. Further,each of the one or more pieces of previous SV information can includeprevious side information (or additional information) such as a stringsize, a string location, and/or the like of the respective previouslycoded string. The one or more pieces of previous SV information in theHSVP buffer can include SV prediction (SVP) candidates or HSVPcandidates for the current string. The HSVP buffer (e.g., a table) canbe maintained during an encoding/a decoding process. The HSVP buffer canbe used in any suitable encoder and/or decoder.

According to aspects of the disclosure, the HSVP buffer can be aseparate history buffer that stores the one or more pieces of previousSV information in a suitable order, such as a decoding order, a reversedecoding order, or a pre-defined order. Entries (e.g., the one or morepieces of previous SV information) in the HSVP buffer can be used topredict the current string to be reconstructed in the string copy mode.In an example, the HSVP buffer is separate from other history buffer(s),such as a HMVP buffer, a HBVP buffer, and the like.

To determine whether to add a new entry (e.g., a new piece of SVinformation) into the HSVP buffer, the new piece of SV information canbe compared with existing entries (e.g., the one or more pieces ofprevious SV information) in the HSVP buffer. In an example, the newpiece of SV information can be compared with each of the existingentries in the HSVP buffer. When the new piece of SV information isdetermined to be unique, the new piece of SV information can be added(e.g., stored) into the HSVP buffer. The uniqueness (e.g., whether thenew piece of SV information is different from the existing entries inthe HSVP) of the new piece of SV information can be determinedseparately. The uniqueness of the new piece of SV information can bedetermined based on a new SV of the new piece of SV information and oneor more previous SVs of the one or more pieces of previous SVinformation.

In an example, if a respective vector difference (or a SV difference)between the new SV and each of the one or more previous SVs satisfies acondition (e.g., the respective vector difference is larger than athreshold), the new piece of SV information is determined to be uniqueor different from the one or more pieces of SV information.

In an example, a vector difference does not satisfy the condition (e.g.,the vector difference is not larger than the threshold), new sideinformation (e.g., a new string size) of the new piece of SV informationis further compared with the previous side information of the one ormore pieces of previous SV information. For example, the new string sizeis compared with each string size of the one or more pieces of previousSV information. If a string size difference between the new string sizeand each string size of the one or more pieces of previous SVinformation satisfies a size condition (e.g., the string size differenceis larger than a size threshold), the new string size is determined tobe different from the string size(s) of the one or more pieces ofprevious SV information. Thus, the new SV information is determined tobe different from the one or more pieces of SV information.

If the new piece of SV information is different from each of theexisting entries in the HSVP buffer, the new piece of SV information canbe determined to be unique. Alternatively, to determine whether to add anew entry into the HSVP buffer, the new piece of SV information can becompared with a subset of the existing entries (e.g., a subset of theone or more pieces of previous SV information) in the HSVP buffer,similarly as described above. For example, the subset of the existingentries includes first N1 entries in the HSVP buffer where N1 is apositive integer that is less than a number of entries in the HSVPbuffer. When the new piece of SV information is determined to be unique,as described above, the new piece of SV information can be stored intothe HSVP buffer.

In some examples, when the new piece of SV information is stored intothe HSVP buffer, one of the existing entries in the HSVP buffer isremoved.

The HSVP buffer can be reset (e.g., emptied) when a new CTU (or a newCTB) row is encountered.

Similar to the CBVP, a class-based history-based string vectorprediction (CSVP) can be established, for example, to predict a currentstring. In the CSVP, one or more pieces of previous SV information in aHSVP buffer that meet certain conditions can be classified intocorresponding categories (or classes), and thus forming a CSVP buffer.In an example, each piece of previous SV information in the HSVP bufferis for a previously coded string that is decoded prior to the currentblock. The piece of previous SV information for the previously codedstring can include a SV, a string size, a string position, and/or thelike.

A plurality of categories or classes can be built, for example, based onone or more of: a string size, an occurrence of a SV, a string position,and the like, similar as described above with reference to a CBVPbuffer. In an example, the plurality of categories is built based on oneor more of: the string size and the occurrence of a SV. An index can beused to point to a first entry in a particular class to select an SVpredictor from the plurality of classes.

According to aspects of the disclosure, if the previous SV informationfor the previously coded string meets the following conditions, theprevious SV information can be classified into the correspondingcategories (or classes).

-   -   (i) Class 0: the string size is greater than or equal to a        threshold.    -   (ii) Class 1: an occurrence (or a frequency) of the SV is        greater than or equal to 2. The occurrence of the SV refers to a        number of times the SV is used to predict previously coded        string(s). When a pruning process is used to form the CSVP        buffer, the SV can be stored in one entry (instead of in        multiples entries having the same SV) when the SV is used        multiple times in predicting previously coded strings. In an        example, the occurrence of the SV is recorded.

The CSVP buffer can be constructed by appending the SV predictor(s) ofeach category in an order from Class 0 to Class 6. As described above,an index (e.g., a flag) can be used to point to a first entry in aparticular class to select an SV predictor from the plurality ofclasses. In some examples, when the index is signaled, the SV predictorcan be determined to be an SV for the current string. Otherwise, whenthe index is not signaled, the SV of the current string can be codeddirectly.

According to aspects of the disclosure, coding information for a currentblock of a current picture can be decoded. The coding information canindicate that a coding mode for the current block is one of: the IBCmode and the string copy mode.

A history buffer can be configured to store vector information ofpreviously decoded units of samples in the current picture. In anexample, the previously decoded units of samples include a blockpreviously decoded in the IBC mode and a string previously decoded inthe string copy mode. The previously decoded units of samples can bedecoded prior to the current block. The vector information can beincluded as entries of the history buffer. Each of the vectorinformation can include a vector (e.g., a BV, an SV) used to predict acorresponding one of the previously decoded units of samples. In someexamples, each of the vector information further includes additionalinformation (or side information) of the one of the previously decodedunits of samples, such as a unit size, a unit location of the one of thepreviously decoded units of samples. Each of the previously decodedunits of samples can be a block (e.g., a CB) or a string.

Current vector information for a current unit of samples in the currentblock can be determined based on the coding mode for the current block.In an example, the current vector information is determined based on thecoding mode for the current block and the history buffer. The currentvector information can include a current vector used to predict thecurrent unit of samples. If the coding mode of the current block is theIBC mode, the current vector is a current BV for the current block. Ifthe coding mode is the string copy mode, the current vector is a currentSV for a current string in the current block. The current unit ofsamples can be reconstructed based on the current vector informationincluding, for example, the current vector.

According to aspects of the disclosure, the history buffer can be ajoint buffer to store HBVP candidate(s) for previously coded block(s)and HSVP candidate(s) for previously coded string(s). The HSVPcandidate(s) (e.g., SV(s)) and HBVP candidate(s) (e.g., BV(s)) can bestored in a suitable order, such as a decoding order, a reverse decodingorder, or a pre-defined order in the history buffer. The entries in thehistory buffer can be used to predict the current vector information forthe current unit of samples in the current block, such as the current SVif the coding mode is the string copy mode or the current BV if thecoding mode is the IBC mode.

In an embodiment, the coding mode for the current block is the IBC mode,and the current unit of samples is the current block. Accordingly, a BVpredictor candidate list for the current block can be determined basedat least on the vector information in the history buffer, and thecurrent BV can be determined based on the BV predictor candidate listwhere the current vector information includes the current BV. The BVpredictor candidate list can include the BV(s) and/or the SV(s) in thehistory buffer. The BV predictor candidate list can further includespatial candidate(s) of the current block.

When the current block is predicted using the merge BV prediction modeor the skip mode vector prediction, the current BV is predicted from theBV predictor candidate list. For example, the current BV is one of theBV(s) and the SV(s) in the history buffer. When the current block ispredicted using the non-merge BV prediction mode, the current BV ispredicted from the BV predictor candidate list and a vector differencebetween the current BV and a BV candidate (e.g., one of the BV(s) andthe SV(s) in the history buffer) in the BV predictor candidate list.

In an embodiment, the coding mode for the current block is the stringcopy mode. Thus, the current unit of samples is the current string inthe current block, and the current SV is for the current string. Thecurrent SV for the current string can be determined from the vectorinformation in the history buffer. An index (e.g., a flag) can besignaled to point to an entry (e.g., one of the vector information) inthe history buffer. If the entry is for the block previously decoded inthe IBC mode, the current SV for the current string is the BV used topredict the block previously decoded in the IBC mode. If the entry isfor the string previously decoded in the string copy mode, the currentSV for the current string is the SV used to predict the stringpreviously decoded in the string copy mode. Alternatively, the currentSV for the current string can be coded directly, and thus can bedetermined directly, for example, from the coding information when noindex is signaled to indicate which entry in the history buffer is to beused to predict the current SV.

According to aspects of the disclosure, whether to add the currentvector information into the history buffer can be determined based onthe current vector information and one or more of the vector informationin the history buffer. When the current vector information is differentfrom the one or more of the vector information, the current vectorinformation can be stored in the history buffer. When the current vectorinformation is different from the one or more of the vector information,the current vector information is considered to be unique. The one ormore of the vector information can include (i) a subset (e.g., first N2entries) of the vector information in the history buffer or (ii) thevector information in the history buffer. N2 can be a positive integerthat is less than a number of pieces of information in the vectorinformation in the history buffer.

The uniqueness of the current vector information can be determined basedon vector difference(s) between the current vector and respectivevector(s) (e.g., SV(s) and/or BV(s)) of the one or more of the vectorinformation in the history buffer. In an example, each of the one ormore of the vector information includes a previous vector that is a BVor a SV. If a difference (also referred to as a vector difference)between the current vector and each previous vector of the one or moreof the vector information in the history buffer satisfies a condition(e.g., the difference is larger than a pre-determined threshold), thecurrent vector information can be determined to be unique or differentfrom the one or more of the vector information. Thus, the current vectorinformation can be stored into the history buffer. Otherwise, if thedifference between the current vector and a previous vector of one ofthe one or more of the vector information does not satisfy the condition(e.g., the difference is not larger than the pre-determined threshold),the current vector information can be determined not to be differentfrom the one of the one or more of the vector information, and thus thecurrent vector information is not unique. In an example, the currentvector information that is not unique is not stored into the historybuffer. In an example, an occurrence of the previous vector of the oneof the one or more of the vector information is increased by 1 and theoccurrence is recorded.

The current vector information can include additional currentinformation, such as a current unit size, a current unit location,and/or the like of the current unit of samples. The current unit sizecan indicate a number of samples in the current unit of samples. The oneor more of the vector information in the history buffer can includeadditional previous information, such as previous unit size(s), previousunit location(s) of the corresponding one or more of the previouslydecoded units of samples in the current picture. Each previous unit sizecan indicate a number of samples in the respective previous decoded unitof samples.

In an embodiment, the uniqueness of the current vector information canbe determined based on the additional current information of the currentunit of samples and the additional previous information of the one ormore of the previously decoded units of samples. For example, thecurrent vector information is determined to be different from the one ormore of the vector information if a size difference between the currentunit size and each respective previous unit size of the one or more ofthe vector information is larger than a pre-determined size threshold.

In an embodiment, in addition to the vector difference(s) between thecurrent vector and the respective previous vector(s) (e.g., SV(s) and/orBV(s)) of the one or more of the vector information, the uniqueness ofthe current vector information can be determined further based on theadditional current information of the current unit of samples and theadditional previous information of the one or more of the previouslydecoded units of samples. For example, when a difference between thecurrent vector and a previous vector of one of the one or more of thevector information in the history buffer is not larger than thepre-determined threshold, the current vector information is determinedto be different from the one or more of the vector information if thesize difference between the current unit size and each respectiveprevious unit size of the one or more of the vector information islarger than the pre-determined size threshold. Further, the currentvector information can be stored in the history buffer, and the one ofthe one or more of the vector information can be removed from thehistory buffer. Alternatively, the current vector information canreplace the one of the one or more of the vector information in thehistory buffer. In an example, an occurrence of the previous vector ofthe one of the one or more of the vector information is increased by 1and the occurrence is recorded as the occurrence of the current vectorof the current vector information.

According to an aspect of the disclosure, the vector information in thehistory buffer can include previous vectors, previous unit sizes, andprevious unit locations of the corresponding previously decoded unit ofsamples. Thus, each of the vector information can be classified into oneof a plurality of categories (or classes) based on but not limited to anoccurrence of the previous vector, the previous unit size, the previousunit location, and/or the like. The occurrence of the previous vectorcan refer to a number of times that the previous vector is used topredict one or more previously decoded units of samples. In an example,the same previous vector is used to predict two previously decoded unitsof samples, and thus the corresponding one of the vector informationincludes the previous vector, the previous unit size, and the previousunit location of the most recently decoded units of samples. Theoccurrence of the previous vector is 2. In an example, each of thevector information can be classified into one of a plurality ofcategories based on the occurrence of the previous vector and theprevious unit size. Accordingly, the history buffer can be a class-basedhistory buffer, similar to that described above with reference to theCBVP and CSVP. The class-based history buffer can be a joint buffer thatincludes CBVP candidate(s) and CSVP candidate(s). In an example, thecoding information for the current block further includes an index(e.g., a flag). The current vector information can be determined to bean entry (e.g., a first entry) in one of the plurality of categorieswhere the one of the plurality of categories can be indicated by theindex.

According to an aspect of the disclosure, one of the vector informationin the history buffer can include a string vector and one of (i) astring location and (ii) a string size of the string previously decodedin the string copy mode. The string location can be a location of apre-determined sample in the string previously decoded in the stringcopy mode. The pre-determined sample can be a first sample, a lastsample, or any suitable sample in a scanning order (e.g., a decodingorder, a reverse decoding order, or the like). The string size can be anumber of samples in the string previously decoded in the string copymode.

According to aspects of the disclosure, coding information for a currentblock can be decoded where the coding information can indicate that thecurrent block is coded in the string copy mode. In an example, thecurrent block includes one or more strings.

A SV and a string length of a current string in the current block can bedetermined based on the coding information. In an example, the currentstring is one of the one or more strings. The string length can be anysuitable integer, such as in a range from 1 to (a block size−1) wherethe block size (e.g., a block area) is a number of samples in thecurrent block. The block size can be equal to a block width multipliedby a block height. According to an aspect of the disclosure, the stringlength can be one or a multiple of a positive integer L, for example,the string length is equal to N3 times L. N3 is a positive integer and Lis larger than 1. In an example, L is less than or equal to the blocksize of the current block divided by 2. For example, the string lengthcan be L, 2 L, 3 L, or the like. If L is 4, the string length is 4, 8,12, or the like.

The current string can be reconstructed based on the SV and the stringlength of the current string.

In an example, the current block is a luma block and L is 4 or 4 lumasamples.

In an example, the current block is a chroma block. A chroma subsamplingformat is 4:2:0 indicating that the chroma block has half a height andhalf a width of a corresponding luma block. If the chroma block is codedjointly with the corresponding luma block, L is 2 or 2 chroma samples.If the chroma block is coded separately with the corresponding lumablock, L is 4 or 4 chroma samples.

In an example, the current block further includes escape samples thatare outside of the one or more strings. The escape samples are notpredicted using reference string(s) indicated by corresponding SV(s).The escape samples can be decoded directly. A number of the escapesamples can be one or a multiple of L In an example, a number of escapesamples in a same row of the current block is one or a multiple of L,such as L, 2 L, or the like.

According to aspects of the disclosure, the coding information canfurther include a syntax element (or a length syntax element) indicatingthe string length. Instead of coding the actual string length (e.g., 8samples) of the current string, the string length can be coded where thelength syntax element has a coded value (e.g., 2) that is equal to thestring length divided by L (e.g., 4).

After decoding the syntax element, the coded value (e.g., 2) of thesyntax element can be multiplied by L to recover the string length(e.g., 8).

The coded value of the syntax element can be an integer that is in arange from 1 to (M1/L−1), and M1 can be the block size (e.g., the numberof samples in the current block). For example, if M1 is 256, L is 4, andthus M1/L is 64 and the range is from 1 to 63. Accordingly, the codedvalue of the syntax element can be 1, 2, . . . , or 63 indicating thestring length to be 4, 8, . . . , or 252, respectively. Thus, by codingthe value of the syntax element as one or multiples of L, the range forthe coded value of the syntax element can be reduced from a range of Lto (M1−L) to the range of 1 to (M1/L−1). For example, when M1 is 256 andL is 4, the range of the value of the syntax element can be reduced froma range of 4 to 252 (with a step of 4) to the range of 1 to 63 (with astep of 1), and thus coding efficiency can be increased.

Multiple resolutions (or multiple precisions) can be used in coding theSV. A syntax element (such as a resolution syntax element, an indicator)can be signaled in a video bitstream to indicate with which resolutionthe SV is coded from a predefined set of resolutions. In an example, theresolution syntax element that indicates the resolution used for the SVcan be decoded from the coding information.

In an embodiment, the predefined set of resolutions includes tworesolutions: (i) 1-sample (1-pel) resolution and (ii) 4-sample (4-pel)resolution, and thus the resolution used for the SV can be chosen fromthe 1-pel resolution or the 4-pel resolutions using the resolutionsyntax element (such as a 1-bit indicator) having 1 bit. If the 4-pelresolution is chosen, both an x component and a y component of the SVare one or multiples of 4. Thus, a decoded SV (or an intermediate SVwhen no prediction is used) or a SV difference (or an intermediate SVdifference) can be left shifted by 2 bits to reconstruct the SV (or thereal SV). The operation of a left shift by 2 bits is equivalent tomultiplying the intermediate SV or the intermediate SV difference by 4.

A string size (or a string length) can represent a number of samples ina string in a block (e.g., a CB or a CU). In an example, the string sizeis a number from 1 up to (M1−1) where M1 is the block size (e.g., anumber of samples in the block). In an example, as described above, thestring size is one or a multiple of L and is in a range from L to (M1−L)with a step of L. Length information indicating the respective stringsize can be signaled for each coded string in the block. According toaspects of the disclosure, last length information of a last codedstring (e.g., a string that is coded last) in the block does not need tobe signaled. In an example, referring to FIG. 14 , the string (1430) iscoded (e.g., encoded and decoded) first followed by the string (1431),and thus the string (1431) is the last coded string in the current block(1435). Alternatively, if the string (1431) is coded first followed bythe string (1430), the string (1430) is the last coded string in thecurrent block (1435). The last length information can indicate a laststring size (or a last string length) of the last coded string. Thus,the last string size of the last coded string is not signaled.

When the last string size of the last coded string is not signaled, thelast string size can be inferred from a number of already coded samplesin the block. The last string size can be inferred from the number ofsamples in the block and the number of already coded samples in theblock. In an example, the block includes a plurality of strings, thenumber of already coded samples in the block is determined from one ormore string lengths of one or more remaining strings in the plurality ofstrings. Accordingly, the last string length can be determined based on(i) the number of samples in the current block and (ii) the one or morestring lengths of the one or more remaining strings in the plurality ofstrings.

For example, referring to FIG. 14 , the current block (1435) includesthe two strings: the strings (1430) and (1431). The string (1430) iscoded first and the string (1431) is coded last after coding the string(1430). The string (1431) is the last coded string in the current block(1435) and thus a last string length of the string (1431) does not needto be coded. The last string length can be inferred from the block size(e.g., 64 samples) and a number of already coded samples in the currentblock (1435). In the example of FIG. 14 , the number of the alreadycoded samples in the current block (1435) is the string size (e.g., 29)of the string (1430). The last string length can be inferred to be equalto the block size (e.g., 64) minus the number (e.g., 29) of the alreadycoded samples in the current block (1435), and thus is 35 samples.

A flag (e.g., a last-coded string flag) can be signaled for one of theplurality of strings in the block to indicate whether the one of theplurality of strings is a last coded string in the block (e.g., a stringthat is coded last in the block). If the flag is true, the one of theplurality of strings is the last coded string in the block. A stringsize of the one of the plurality of strings (e.g., the last codedstring) does not need to be signaled.

A flag (e.g., a last-coded string flag) can be signaled for each of theplurality of strings in the block. Referring to FIG. 14 , a first flag(or a first last-coded string flag) is signaled for the string (1430)and a second flag (or a second last-coded string flag) is signaled forthe string (1431). The first flag is false indicating that the string(1430) is not the last coded string, and the second flag is trueindicating that the string (1431) is the last coded string.

The flag for the one of the plurality of strings can be context coded,for example, using context-adaptive binary arithmetic coding (CABAC).The context modeling of the flag can depend on a string position of theone of the plurality of strings. As described above, the string positioncan be represented by a position of a sample (e.g., a first sample in adecoding order) in the one of the plurality of strings.

The context modeling of the flag can depend on a number of remainingsamples in the block where the remaining samples are coded before theone of the plurality of strings.

In an example, the flag does not need to be signaled if the one of theplurality of strings is the first string in the block.

FIG. 15 shows a flow chart outlining a process (1500) according to anembodiment of the disclosure. The process (1500) can be used toreconstruct a block or a string in a picture of a coded video sequence.The process (1500) can be used in the reconstruction of the block togenerate a prediction block for the block under reconstruction. The termblock in the disclosure may be interpreted as a prediction block, a CB,a CU, or the like. In various embodiments, the process (1500) areexecuted by processing circuitry, such as the processing circuitry inthe terminal devices (310), (320), (330) and (340), the processingcircuitry that performs functions of the video encoder (403), theprocessing circuitry that performs functions of the video decoder (410),the processing circuitry that performs functions of the video decoder(510), the processing circuitry that performs functions of the videoencoder (603), and the like. In some embodiments, the process (1500) isimplemented in software instructions, thus when the processing circuitryexecutes the software instructions, the processing circuitry performsthe process (1500). The process starts at (S1501) and proceeds to(S1510).

At (S1510), coding information for a current block of a current picturecan be decoded. The coding information can indicate a coding mode forthe current block being one of: the IBC mode and the string copy mode.

At (S1520), current vector information for a current unit of samples inthe current block can be determined based on the coding mode for thecurrent block and a history buffer. The history buffer can be configuredto store vector information of previously decoded units of samples inthe current picture. In an example, the previously decoded units ofsamples include a block previously decoded in the IBC mode and a stringpreviously decoded in the string copy mode. In an example, thepreviously decoded units of samples are decoded prior to the currentblock. The history buffer can be a joint buffer that combines a HSVPbuffer and a HBVP buffer.

The vector information can be stored as entries of the history buffer.Each of the vector information can include a vector (e.g., a BV, an SV)used to predict a corresponding one of the previously decoded units ofsamples. In some examples, each of the vector information furtherincludes additional information (or side information) of the one of thepreviously decoded units of samples, such as a unit size, a unitlocation of the one of the previously decoded units of samples. Each ofthe previously decoded units of samples can be a block (e.g., a CB) or astring.

The current vector information can include a current vector used topredict the current unit of samples. If the coding mode of the currentblock is the IBC mode, the current unit of samples is the current block,and the current vector is a current BV for the current block. If thecoding mode is the string copy mode, the current unit of samples is acurrent string in the current block, and the current vector is a currentSV for the current string in the current block.

At (S1530), the current unit of samples can be reconstructed based onthe current vector information including, for example, the currentvector.

The process (1500) can be suitably adapted. Step(s) in the process(1500) can be modified and/or omitted. Additional step(s) can be added.Any suitable order of implementation can be used. For example, when thecurrent vector information is determined to be unique, as describedabove, the current vector information can be stored into the historybuffer. In some examples, a pruning process is used and one of thevector information in the history buffer is removed when the currentvector information is stored into the history buffer.

FIG. 16 shows a flow chart outlining a process (1600) according to anembodiment of the disclosure. The process (1600) can be used toreconstruct a string in a current block in a picture of a coded videosequence. The process (1600) can also be used in the reconstruction ofthe current block to generate a prediction block for the current blockunder reconstruction. The term block in the disclosure may beinterpreted as a prediction block, a CB, a CU, or the like. In variousembodiments, the process (1600) are executed by processing circuitry,such as the processing circuitry in the terminal devices (310), (320),(330) and (340), the processing circuitry that performs functions of thevideo encoder (403), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the video decoder (510), the processing circuitry thatperforms functions of the video encoder (603), and the like. In someembodiments, the process (1600) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1600). The process startsat (S1601) and proceeds to (S1610).

At (S1610), coding information for the current block can be decoded. Thecoding information can indicate that the current block is coded in thestring copy mode. In an example, the current block includes one or morestrings.

At (S1620), a SV and a string length of a current string in the currentblock can be determined based on the coding information. The currentstring can be one of the one or more strings. The string length can beone or a multiple of the positive integer L that is larger than 1, andthus the string length can be equal to N3 times L. As described above,N3 is a positive integer.

In an example, the current block is a luma block and L is 4.

In an example, the current block is a chroma block and a chromasubsampling format is 4:2:0. If the chroma block is coded jointly with acorresponding luma block, L is 2. If the chroma block is codedseparately from the corresponding luma block, L is 4.

In an example, the current block further includes escape samples thatare outside of the one or more strings. A number of the escape samplescan be one or a multiple of L.

In an example, the coding information further includes a syntax elementindicating the string length. A coded value of the syntax element is thestring length divided by L. The coded value of the syntax element can bean integer in a range from 1 to (M1/L−1) where M1 is a number of samplesin the current block. The string length can be determined to be thevalue of the syntax element multiplied by L.

At (S1630), the current string can be reconstructed based on the SV andthe string length of the current string.

The process (1600) can be suitably adapted. Step(s) in the process(1600) can be modified and/or omitted. Additional step(s) can be added.Any suitable order of implementation can be used. For example, thecoding information further includes a syntax element that indicates aresolution used for the SV. In an example, the syntax element has 1 bitindicating that the resolution for the SV is 1-pel or 4-pel. If theresolution for the SV is 4-pel, an intermediate SV can be determinedfrom the coding information, and the SV can be determined to be theintermediate SV multiplied by 4.

In an example, the current block includes a plurality of strings. A laststring length of a last string to be coded in the plurality of stringsis not signaled. The last string length can be determined based on (i)the number of samples in the current block and (ii) one or more stringlengths of one or more remaining strings in the plurality of strings.The coding information can include a flag indicating whether the currentstring is the last string.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 17 shows a computersystem (1700) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 17 for computer system (1700) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1700).

Computer system (1700) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1701), mouse (1702), trackpad (1703), touchscreen (1710), data-glove (not shown), joystick (1705), microphone(1706), scanner (1707), camera (1708).

Computer system (1700) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1710), data-glove (not shown), or joystick (1705), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1709), headphones(not depicted)), visual output devices (such as screens (1710) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability-some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)) andprinters (not depicted).

Computer system (1700) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1720) with CD/DVD or the like media (1721), thumb-drive (1622),removable hard drive or solid state drive (1723), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1700) can also include an interface (1754) to one ormore communication networks (1755). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CANBus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general purpose data ports or peripheral buses (1749) (such as,for example USB ports of the computer system (1700)); others arecommonly integrated into the core of the computer system (1700) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (1700) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1740) of thecomputer system (1700).

The core (1740) can include one or more Central Processing Units (CPU)(1741), Graphics Processing Units (GPU) (1742), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1743), hardware accelerators for certain tasks (1744), graphics adapter(1750), and so forth. These devices, along with Read-only memory (ROM)(1745), Random-access memory (1746), internal mass storage such asinternal non-user accessible hard drives, SSDs, and the like (1747), maybe connected through a system bus (1748). In some computer systems, thesystem bus (1748) can be accessible in the form of one or more physicalplugs to enable extensions by additional CPUs, GPU, and the like. Theperipheral devices can be attached either directly to the core's systembus (1748), or through a peripheral bus (1749). In an example, a display(1710) can be connected to the graphics adapter (1750). Architecturesfor a peripheral bus include PCI, USB, and the like.

CPUs (1641), CPUs (1742), FPGAs (1743), and accelerators (1744) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1745) or RAM (1746). Transitional data can be also be stored in RAM(1746), whereas permanent data can be stored for example, in theinternal mass storage (1747). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1741), GPU (1742), massstorage (1747), ROM (1745), RAM (1746), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1700), and specifically the core (1740) can providefunctionality as a result of processor(s)(including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1740) that are of non-transitorynature, such as core-internal mass storage (1747) or ROM (1745). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1740). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1740) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1746) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1744)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   -   JEM: joint exploration model    -   VVC: versatile video coding    -   BMS: benchmark set    -   MV: Motion Vector    -   HEVC: High Efficiency Video Coding    -   MPM: most probable mode    -   WAIP: Wide-Angle Intra Prediction    -   SET: Supplementary Enhancement Information    -   VUI: Video Usability Information    -   GOPs: Groups of Pictures    -   TUs: Transform Units,    -   PUs: Prediction Units    -   CTUs: Coding Tree Units    -   CTBs: Coding Tree Blocks    -   PBs: Prediction Blocks    -   HRD: Hypothetical Reference Decoder    -   SDR: standard dynamic range    -   SNR: Signal Noise Ratio    -   CPUs: Central Processing Units    -   GPUs: Graphics Processing Units    -   CRT: Cathode Ray Tube    -   LCD: Liquid-Crystal Display    -   OLED: Organic Light-Emitting Diode    -   CD: Compact Disc    -   DVD: Digital Video Disc    -   ROM: Read-Only Memory    -   RAM: Random Access Memory    -   ASIC: Application-Specific Integrated Circuit    -   PLD: Programmable Logic Device    -   LAN: Local Area Network    -   GSM: Global System for Mobile communications    -   LTE: Long-Term Evolution    -   CANBus: Controller Area Network Bus    -   USB: Universal Serial Bus    -   PCI: Peripheral Component Interconnect    -   FPGA: Field Programmable Gate Areas    -   SSD: solid-state drive    -   IC: Integrated Circuit    -   CU: Coding Unit    -   PDPC: Position Dependent Prediction Combination    -   ISP: Intra Sub-Partitions    -   SPS: Sequence Parameter Setting

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder,comprising: decoding coding information for a current block, the codinginformation indicating that the current block is coded in a string copymode and the coding information including a syntax element indicating astring length of a current string in the current block; determining astring vector (SV) and the string length of the current string in thecurrent block based on the coding information, the string length beingN3 times L, N3 and L being positive integers, and L being larger than 1;and reconstructing the current string based on the SV and the stringlength of the current string, wherein a coded value of the syntaxelement indicates the string length divided by L, the string length isconstrained to a multiple of L, and the coded value of the syntaxelement is one of (M1/L−1) values, each of the (M1/L−1) valuescorresponding to a different multiple of L, and M1 being a number ofsamples in the current block.
 2. The method of claim 1, wherein thecurrent block is a luma block and L is
 4. 3. The method of claim 1,wherein the current block is a chroma block; a chroma subsampling formatis 4:2:0 indicating that the chroma block has half a height and half awidth of a corresponding luma block; L is 2 based on the chroma blockbeing coded jointly with the corresponding luma block; and L is 4 basedon the chroma block being coded separately from the corresponding lumablock.
 4. The method of claim 1, wherein the current block includes oneor more strings that has the current string; the current block furtherincludes escape samples outside of the one or more strings; and a numberof the escape samples is constrained to one or a multiple of L.
 5. Themethod of claim 4, wherein a number of escape samples in a same row ofthe current block is constrained to one or a multiple of L.
 6. Themethod of claim 1, wherein the coded value of the syntax element is aninteger in a range from 1 to (M1/L−1).
 7. The method of claim 1, whereinthe decoding the coding information further includes decoding the syntaxelement; and the determining the string length further includesdetermining the string length to be the coded value of the syntaxelement multiplied by L.
 8. The method of claim 1, wherein the decodingthe coding information further includes decoding another syntax elementthat indicates a resolution used for the SV.
 9. The method of claim 8,wherein the other syntax element has 1 bit indicating that theresolution for the SV is one of (i) 1-sample and (ii) 4-sample; andbased on the resolution for the SV being 4-sample, the determining theSV further includes: determining an intermediate SV from the codinginformation; and determining the SV to be the intermediate SV multipliedby
 4. 10. The method of claim 1, wherein the current block includes aplurality of strings that has the current string and a last stringlength of a last string that is to be coded in the plurality of stringsis not signaled.
 11. The method of claim 10, wherein the last stringlength is determined based on (i) a number of samples in the currentblock and (ii) one or more string lengths of one or more remainingstrings in the plurality of strings.
 12. The method of claim 10, whereinthe coding information includes a flag indicating whether the currentstring is the last string.
 13. An apparatus for video decoding,comprising processing circuitry configured to: decode coding informationfor a current block, the coding information indicating that the currentblock is coded in a string copy mode and the coding informationincluding a syntax element indicating a string length of a currentstring in the current block; determine a string vector (SV) and thestring length of the current string in the current block based on thecoding information, the string length being N3 times L, N3 and L beingpositive integers, and L being larger than 1; and reconstruct thecurrent string based on the SV and the string length of the currentstring, wherein a coded value of the syntax element indicates the stringlength divided by L, the string length is constrained to a multiple ofL, and the coded value of the syntax element is one of (M1/L−1) values,each of the (M1/L−1) values corresponding to a different multiple of L,and M1 being a number of samples in the current block.
 14. The apparatusof claim 13, wherein the current block is a luma block and L is
 4. 15.The apparatus of claim 13, wherein the coded value of the syntax elementis an integer in a range from 1 to (M1/L−1).
 16. The apparatus of claim13, wherein the processing circuitry is configured to: decode the syntaxelement; and determine the string length to be the coded value of thesyntax element multiplied by L.
 17. The apparatus of claim 13, whereinthe processing circuitry is configured to: decode another syntax elementthat indicates a resolution used for the SV.
 18. The apparatus of claim13, wherein the current block includes a plurality of strings that hasthe current string and a last string length of a last string that is tobe coded in the plurality of strings is not signaled.